This invention relates to cooling of turbine rotor disks and blades of gas turbine engines with cooling air supplied to a dovetail slot which retains a blade root in a rim of a rotating turbine disk and, in particular, to a cooling air slot which directs cooling air to the dovetail slot.
In gas turbine engines, fuel is burned within a combustion chamber to produce hot gases of combustion. The gases are expanded within a turbine section producing a gas stream across alternating rows of stationary stator vanes and turbine rotor blades to produce usable power. Gas stream temperatures at the initial rows of vanes and blades commonly exceed 2,000 degrees Fahrenheit. Blades and vanes, susceptible to damage by the hot gas stream, are cooled by air compressed upstream within the engine and flowed to the turbine components. One technique for cooling rotating turbine disk assemblies, having blades attached to rims of disks, injects cooling air from stationary cavities within the engine to a disk assembly for distribution to the interior of the turbine blades. A cooling air injection nozzle is a well-known device used to receive compressed air from a compressor of the engine and inject the cooling air through circumferentially spaced passages that impart a swirling movement and directs an injected stream of the cooling air tangentially to the rotating turbine disk assembly. A typical turbine disk assembly has the turbine blades attached to the rims of the disk and a disk side plate attached to a forward or aft face of the disk forming a cooling air passage between the plate and the disk. The plate also is used to axially retain the blades in dovetail slots in the rim of the disk and to support one or more rotating seals. In order to perform these functions, the disk side plate is usually restrained axially and supported radially by the disk out near the rim or on the web, where the stress fields are typically high. In the case where a disk side plate supports inner and outer rotating seals, or where the outer section of the disk side plate requires more radial support, a means of axial retention and radial support may be required at a lower radially inner position of the disk also.
The dovetail slots are circumferentially disposed between posts of the rims. Cooling air flows through radially extending cooling air slots in the rim between the posts or between blade retainer flanges of the posts. The cooling air slots extend to the dovetail slots and thus direct cooling air into the dovetail slots through which cooling air passages in the turbine blades receive the cooling air. The cooling air slots are usually milled in the disk rim and into a hoop stress path of the disk. Stress increases in this region significantly impacts the overall life of the part due to low cycle fatigue. Due to the high stress concentrations seen in this area, the cooling air slot shape is extremely sensitive to small variations in depth, radius, position and its overall alignment to the stress field.
The air slot is typically manufactured by milling a straight slot cut in the radial direction. Such a cooling air slot design has stress peaks in a fillet face, top and bottom breakout locations, and a dovetail slot bottom break-edge. It is undesirable to have the stress peak in the fillet face or the breakout locations, because these locations are hard to measure and control in the manufacturing process. This may lead to a non-robust design because it is very sensitive to slight manufacturing variations. Also, the high peak stress in these areas leads to a low life due to low cycle fatigue.
In some engines, the cooling air slot may be the life limiting feature of the part. By way of example, the CFM56-5B, -5C and -7 engines models have several calculated life limiting features in the HPT disk. It is desirable to increase the life limit to perhaps 20,000 cycles or more in such an engine. It is highly desirable to have a cooling air slot design with improved durability and one which provides a substantial increase in the overall life of the slot and lowers susceptibility to low cycle fatigue.
A gas turbine engine rotor disk assembly includes a disk having an annular hub circumscribed about a centerline. The disk has an annular web extending radially outwardly from the hub and an annular rim is disposed on a radially outer end of the web. A plurality of dovetail slots extend generally axially through the rim. A plurality of cooling air slots extend generally radially through the rim and are skewed circumferentially with respect to the centerline and slanted axially aftwardly with respect to a normal radius perpendicular to the centerline.
In the exemplary embodiment illustrated herein, each cooling air slot has parallel side walls skewed circumferentially with respect to the centerline and an aft wall extending between the side walls and slanted axially aftwardly with respect to the normal radius which is perpendicular to the centerline. A fillet is formed between each of side walls and the aft wall. Each fillet has a fillet radius of curvature. The aft wall is curved and has a wall radius of curvature. The wall radius is about equal to a width of the cooling air slot between side walls. The wall radius of curvature is about four times larger than the fillet radius of curvature. The side walls are skewed circumferentially about 5 degrees with respect to the centerline and the aft wall is slanted axially aftwardly about 18 degrees with respect to the normal radius which is perpendicular to the centerline.
The axially cutback and circumferentially skewed cooling air slot lowers the stress in the air slot to reduce low cycle fatigue and improve the overall life of the disk. The axially cutback and circumferentially skewed cooling air slot can provide a more robust design due to a decrease in sensitivity to manufacturing variation by shifting the stress peak to the aft wall of the air slot.